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PRE-LAUNCH OPERATIONS
After the Space Shuttle has been rolled out to the launch pad on the
Mobile Launcher Platform (MLP), all pre-launch activities are
controlled from the Launch Control Center (LCC).
After the Shuttle is in place on the launch pad support columns, and
the Rotating Service Structure (RSS) is placed around it, power for
the vehicle is activated. The MLP and the Shuttle are then
electronically and mechanically mated with support launch pad
facilities and ground support equipment. An extensive series of
validation checks verify that the numerous interfaces are functioning
properly.
Meanwhile, in parallel with pre-launch pad activities, cargo
operations get underway in the RSS's Payload Changeout Room.
Vertically integrated payloads are delivered to the launch pad
before the Shuttle is rolled out. They are stored in the Payload
Changeout Room until the Shuttle is ready for cargo loading. Once
the RSS is in place around the orbiter, the payload bay doors are
opened and the cargo is installed. Final cargo and payload bay
closeouts are completed in the Payload Changeout Room and the payload
bay doors are closed for flight.
Pre-launch Propellant Loading. Initial Shuttle propellant loading
involves pumping hypergolic propellants into the orbiter's aft and
forward Orbital Maneuvering System and Reaction Control System
storage tanks, the orbiter's hydraulic Auxiliary Power Units, and SRB
hydraulic power units. These are hazardous operations, and while
they are underway work on the launch pad is suspended.
Since these propellants are hypergolic -- that is they ignite on
contact with one another--oxidizer and fuel loading operations are
carried out serially, never in parallel.
Finally, dewar tanks on the Fixed Service Structure (FSS), are
filled with liquid oxygen and liquid hydrogen, which will be loaded
into the orbiter's Power Reactant and Storage Distribution (PRSD)
tanks during the launch countdown.
Final Pre-launch Activities. Before the formal Space Shuttle launch
countdown starts, the vehicle is powered down while pyrotechnic
devices -- various ordinance components -- are installed or hooked
up. The extravehicular Mobility Units (EMUs) -- space suits -- are
stored On Board along with other items of flight crew equipment.
When closeouts of the Space Shuttle and the launch pad are
completed, all is in readiness for the countdown to get underway.
Launch Control Center. While the VAB can be considered the heart of
LC-39, the Launch Control Center (LCC) can easily be called its brain.
The LCC is a 4-story building connected to the east side of the VAB
by an elevated, enclosed bridge. It houses four firing rooms that
are used to conduct NASA and classified military launches of the
Space Shuttle. Each firing room is equipped with the Launch
Processing System (LPS) which monitors and controls most Shuttle
assembly, checkout and launch operations. Physically, the LCC is 77
ft. high, 378 ft. long and 181 ft. wide.
Thanks to the LPS, the countdown for the Space Shuttle takes only
about 40 hours, compared with the 80 plus hours usually needed for a
Saturn/Apollo countdown. Moreover, the LPS calls for only about 90
people to work in the firing room during launch operations --
compared with about 450 needed for earlier manned missions.
From the outside, the LCC is virtually unchanged from its original
Apollo-era configuration, except that a fourth floor office has been
added to the southwest and northwest corners corner of the building.
The interior of the LCC has undergone extensive modifications to
meet the needs of the Space Shuttle era.
Physically, the LCC is constructed as follows: the first floor is
used for administrative activities and houses the building's
utilities systems control room; the second floor is occupied by the
Control Data Subsystem; the four firing rooms occupy practically all
of the third floor, and the fourth floor, as mentioned, earlier is
used for offices.
During the Shuttle Orbital Flight Test program and the early
operational missions, Firing Room l was the only fully-equipped
control facility available for vehicle checkout and launch. However,
as the Shuttle launch rate increased during the first half of the
1980s, the other three firing rooms were activated. Although NASA
operates the firing rooms, the Department of Defense uses Firing
Rooms 3 and 4 to support its classified, Shuttle-dedicated missions.
Additionally, Firing Room 4 serves as an engineering analysis and
support facility for launch and checkout operations.
Launch Countdown. As experience was gained by launch crews during
the early years of the Space Shuttle program, the launch countdown
was refined and streamlined to the point where the average countdown
now takes a little more than 40 hours. This was not the case early
in the program, when countdowns of 80 hours or more were not uncommon.
The following is a narrative description of the major events of a
typical countdown for the Space Shuttle. The time of liftoff is
predicated on what is called the launch window -- that point in time
when the Shuttle must be launched in order to meet specific mission
objectives such as the deployment of spacecraft at a predetermined
time and location in space.
Launch Minus 3 Days. The countdown gets underway with the
traditional call to stations by the NASA Test Director. This
verifies that the launch team is in place and ready to proceed.
The first item of business is to checkout the backup flight system
and the software stored in the mass memory units and display systems.
Backup flight system software is then loaded into the Shuttle's
fifth general purpose computer (GPC's).
Flight crew equipment stowage begins. Final inspection of the
orbiter's middeck and flight decks are made, and removal of work crew
module platforms begin. Loading preparations for the external tank
get underway, and the Shuttle main engines are readied for tanking.
Servicing of fuel cell storage tanks also starts. Final vehicle and
facility closeouts are made.
Launch Minus 2 Days. The launch pad is cleared of all personnel
while liquid oxygen and hydrogen are loaded into the Shuttle fuel
cell storage tanks. Upon completion, the launch pad area is reopened
and the closeout crew continues its prelaunch preparations.
The orbiter's flight control, navigation and communications systems
are activated. Switches located on the flight and mid- decks are
checked and, if required, mission specialist seats are installed.
Preparations also are made for rollback of the Rotating Service
Structure (RSS).
At launch minus ll hours a planned countdown hold -- called a
built-in hold -- begins and can last for up to 26 hours, 16 minute
depending on the type of payload, tests required and other factors.
This time is used, if needed, to perform tasks in the countdown that
may not have been completed earlier.
Launch Minus 1 Day. Countdown is resumed after the built-in hold
period has elapsed. The RSS is rolled back and remaining items of
crew equipment are installed. Cockpit switch positions are verified,
and oxygen samples are taken in the crew area. The fuel cells are
activated following a fuel cell flow through purge. Communications
with the Johnson Space Center's Mission Control Center (MCC) are
established.
Finally, the launch pad is again cleared of all personnel while
conditioned air that has been blowing through the payload bay and
other orbiter cavities is switched to inert gaseous nitrogen in
preparation for filling the external tank with its super-cold
propellants.
Launch Day. Filling the external tank with liquid oxygen and
hydrogen gets underway. Communications checks are made with elements
of the Air Force's Eastern Space and Missile Center. Gimbal profile
checks of the Orbital Maneuvering System (OMS) engines are made.
Preflight calibration of the Inertial Measurement Units (IMU) is
made, and tracking antennas at the nearby Merritt Island Tracking
Station are aligned for liftoff.
At launch minus 5 hours, 20 minutes -- T minus 5 hours, 20 minutes
-- a 2-hour built-in hold occurs. During this hold, an ice
inspection team goes to the launch pad to inspect the external tank's
insulation to insure that there is no dangerous accumulation of ice
on the tank caused by the super-cold liquids. Meanwhile, the
closeout crew is preparing for the arrival of the flight crew.
Meanwhile, the flight crew, in their quarters at the Operations and
Checkout (O&C) Building, eat a meal and receive a weather briefing.
After suiting up, they leave the O&C Building at about T minus 2
hours, 30 minutes for the launch pad -- the countdown having resumed
at T minus 3 hours.
Upon arriving at the white room at the end of the orbiter access
arm, the crew, assisted by white room personnel, enter the orbiter.
Once on board they conduct air-to-ground communications checks with
the LCC and MCC. Meanwhile, the orbiter hatch is closed and hatch
seal and cabin leak checks are made. The IMU preflight alignment is
made and closed-loop tests with Range Safety are completed. The
white room is then evacuated and the closeout crew proceeds from the
launch pad to a fallback area. At this time, primary ascent guidance
data is transferred to the backup flight system.
At T minus 20 minutes a planned 10-minute hold begins. When the
countdown is resumed on-board computers are commanded to their launch
configuration and fuel cell thermal conditioning begins. Orbiter
cabin vent valves are closed and the backup flight system transitions
into its launch configuration.
At T minus 9 minutes another planned 10-minute hold occurs. Just
prior to resuming the countdown, the NASA Test Director gets the "go
for launch" verification from the launch team. At this point, the
Ground Launch Sequencer (GLS) is turned on and the terminal countdown
starts. All countdown functions are now automatically controlled by
the GLS computer located in the Firing Room Integration Console.
At T minus 7 minutes, 30 seconds, the orbiter access arm is
retracted. Should an emergency occur requiring crew evacuation from
the orbiter, the arm can be extended either manually or automatically
in about 15 seconds.
At T minus 5 minutes, 15 seconds the MCC transmits a command that
activates the orbiter's operational instrumentation recorders. These
recorders store information relating to ascent, on-orbit and descent
performance during the mission. These data are analyzed after
landing.
At T minus 5 minutes, the crew activates the Auxiliary Power Units
(APU) to provide pressure to the Shuttle's three hydraulic systems
which move the main engine nozzles and the aero-aerosurfaces. Also
at this point, the firing circuit for SRB ignition and the range
safety destruct system devices are mechanically enabled by a
motor-driven switch called the safe and arm device.
At about T minus 4 minutes, 55 seconds, the liquid oxygen vent on
the external tank is closed. It had been open to allow the
super-cold liquid oxygen to boil off, thus preventing over
pressurization while the tank remained near its full level. Now,
with the vent closed, preparations are made to bring the tank to its
flight pressure. This occurs at T minus 2 minutes, 55 seconds.
At T minus 4 minutes the final helium purge of the Shuttle's three
main engines is initiated in preparation for engine start. Five
seconds later, the orbiter's elevons, speed brakes and rudder are
moved through a pre-programmed series of maneuvers to position them
for launch. This is called the aerosurface profile.
At T minus 3 minutes, 30 seconds, the ground power transition takes
place and the Shuttle's fuel cells transition to internal power. Up
to this point, ground power had augmented the fuel cells. Then, 5
seconds later, the main engine nozzles are gimballed through a
pre-programmed series of maneuvers to confirm their readiness.
At T minus 2 minutes, 50 seconds, the external tank oxygen vent hood
-- known as the beanie cap -- is raised and retracted. It had been
in place during tanking operations to prevent ice buildup on the
oxygen vents. Fifteen seconds later, at T minus 2 minutes, 35
seconds, the piping of gaseous oxygen and hydrogen to the fuel cells
from ground tanks is terminated and the fuel cells begin to use the
on board reactants.
At T minus 1 minute, 57 seconds, the external tank's liquid hydrogen
is brought to flight pressure by closing the boil off vent, as was
done earlier with the liquid oxygen vent. However, during the
hydrogen boil off of, the gas is piped out to an area adjacent to the
launch pad where it is burned off.
At T minus 31 seconds, the Shuttle's on-board computers start their
terminal launch sequence. Any problem after this point will require
calling a "hold" and the countdown recycled to T minus 20 minutes.
However, if all goes well, only one further ground command is needed
for launch. This is the "go for main engine start," which comes at
the T-minus-10-second point. Meanwhile, the Ground Launch Sequencer
(GLS) continues to monitor more than several hundred launch commit
functions and is able automatically to call a "hold" or "cutoff" if a
problem occurs.
At T minus 28 seconds the SRB booster hydraulic power units are
activated by a command from the GLS. The units provide hydraulic
power for SRB nozzle gimballing. At T minus 16 seconds, the nozzles
are commanded to carry out a pre-programmed series of maneuvers to
confirm they are ready for liftoff. At the same time -- T minus 16
seconds -- the sound suppression system is turned on and water begins
to pour onto the deck of the MLP and pad areas to protect the
Shuttle from acoustical damage at liftoff.
At T minus ll seconds, the SRB range safety destruct system is
activated.
At T minus 10 seconds, the "go for main engine start" command is
issued by the GLS. (The GLS retains the capability to command main
engine stop until just before the SRBs are ignited.) At this time
flares are ignited under the main engines to burn away any residual
gaseous hydrogen that may have collected in the vicinity of the main
engine nozzles. A half second later, the flight computers order the
opening of valves which allow the liquid hydrogen and oxygen to flow
into the engine's turbopumps.
At T minus 6.6 seconds, the three main engines are ignited at
intervals of 120 milliseconds. The engines throttle up to 90 percent
thrust in 3 seconds. At T minus 3 seconds, if the engines are at the
required 90 percent, SRB ignition sequence starts. All of these
split-second events are monitored by the Shuttle's four primary
flight computers.
At T minus zero, the holddown explosive bolts and the T-O umbilical
explosive bolts are blown by command from the on-board computers and
the SRBs ignite. The Shuttle is now committed to launch. The
mission elapsed time is reset to zero and the mission event timer
starts. The Shuttle lifts off the pad and clears the tower at about
T plus 7 seconds. Mission control is handed over to JSC after the
tower is cleared.
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MARSHALL PAYLOAD OPERATIONS CONTROL CENTER
The Payload Operations Control Center (POCC) operated by the NASA's
Marshall Space Flight Center (MSFC), Huntsville, Ala., is the largest
and most diverse of the various POCCs associated with the Space
Shuttle program. Since its functions in many respects parallel those
of other POCCs operated by private industry, the academic community
and government agencies, a description of what it does, how it
operates and who operates it will serve as an overview of this type
of control center.
The Marshall POCC -- like all POCCs -- is a facility designed to
monitor, coordinate, and control on-orbit operation of a Shuttle
payload, particularly Spacelab. During non-mission periods it also
is used for crew training and simulated space operations. It is, in
effect, a command post for payload activities, just as the JSC
Mission Control Center (MCC) is a command post for the flight and
operation of the Space Shuttle.
Both control centers work closely in coordinating mission
activities. In fact, the Marshall POCC originally was housed in
Building 30 at JSC, adjacent to the MCC. It has since been moved to
Building 4663 at Marshall and is an important element of the
Hunstville Operations Support Center (HOSC), which augments the MCC
by monitoring Shuttle propulsion systems.
The Marshall POCC Capabilities Document states that the "POCC
provides physical space, communications, and data system capabilities
to enable user access to payload data (digital, video, and analog),
command uplink, and coordination of activities internal and external
to the POCC."
Members of the Marshall mission management team and principal
investigators and research teams work in the POCC or in adjacent
facilities around-the-clock controlling and directing payload
experiment operations. Using the extensive POCC facilities they are
able to communicate directly with mission crews and direct experiment
activities from the ground. They also can operate experiments and
support equipment on board the Shuttle and manage payload resources.
The POCC operations concept requires a team consisting of the
Payload Mission Manager (PMM) directing the POCC cadre which has
overall responsibility for managing and controlling POCC operations.
Its scientific counterpart, the investigator's operations team, is
the group that conducts, monitors and controls the experiments
carried on the Shuttle, primarily those related to Spacelab.
Generally, POCC operations are carried out by a
management/scientific team of 10 key individuals, headed by the
Payload Operations Director (POD), who is a senior member of the
PMM's cadre. The POD is charged with managing the day-to-day mission
operations and directing the payload operations team and the science
crew.
Other POCC key personnel include:
MISSION SCIENTIST (MSCI) who represents scientists who have
experiments on a specific flight and serves as the interface between
the PMM and the POD in matters relating to mission science operations
and accomplishments.
CREW INTERFACE COORDINATOR (CIC), who coordinates communications
between the POCC and the payload crew.
ALTERNATE PAYLOAD SPECIALIST (APS) is a trained payload specialist
not assigned to flight duty who aids the payload operations team and
the payload crew in solving problems, troubleshooting and modifying
crew procedures, if necessary, and who advises the MSCI on the
possible impact of any problem areas.
PAYLOAD ACTIVITY PLANNER (PAP), who directs mission replanning
activities, as required, and coordinates mission timeline changes
with POCC personnel.
MASS MEMORY UNIT MANAGER (MUM) who sends experiment command uplinks
to the flight crew based on data received from the POCC operations
team.
OPERATIONS CONTROLLER (OC), who coordinates activities of the
payload operations team to insure the efficient accomplishment of
activities supporting real-time execution of the mission timeline.
PAYLOAD COMMAND COORDINATOR (PAYCOM), who configures the POCC for
ground command operation and controls the flow of experiment commands
from the POCC to the flight crew.
DATA MANAGEMENT COORDINATOR (DMC), who is responsible for
maintaining and coordinating the flow of payload experiment data to
and within the POCC the DMC also assesses the impact of proposed
changes to the experiment timeline and payload data requirements that
affect the payload downlink data.
PUBLIC AFFAIRS OFFICER (PAO), who provides mission commentary on
payload activities and serves as the primary source of information on
mission progress to the news media and public.
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SPACE TRACKING AND DATA ACQUISITION
Responsibility for Space Shuttle tracking and data acquisition is
charged to the Goddard Space Flight Center, Greenbelt, Md. This
involves integrating and coordinating all of the worldwide NASA and
Department of Defense tracking facilities needed to support Space
Shuttle missions.
These facilities include the Goddard-operated Ground Network (GN)
and Space Network (SN); the Deep Space Network (DSN) managed for NASA
by the Jet Propulsion Laboratory (JPL), Pasadena, Calif.; the
Ames-Dryden Flight Research Facility, (ADFRC) Edwards, Calif.; and
extensive Department of Defense tracking systems at the Eastern and
Western Space and Missile Centers, as well as the Air Force Satellite
Control Network's (AFSCN) remote tracking stations.
Ground Network. The Ground Network (GN) is a worldwide network of
tracking stations and data-gathering facilities which support Space
Shuttle missions and also maintain communications with low
Earth-orbiting spacecraft. Station management is provided from the
Network Control Center at Goddard. Basically, commands are sent to
orbiting spacecraft from the GN stations and, in return, scientific
data are transmitted to the stations.
The system consists of 12 stations, including three DSN facilities.
GN stations are located at Ascension Island, a British Crown Colony
in the south Atlantic Ocean; Santiago, Chile; Bermuda; Dakar,
Senegal, on the West Coast of Africa; Guam; Hawaii; Merritt Island,
Fla.; Ponce de Leon, Fla.; and the Wallops Flight Facility on
Virginia's Eastern Shore. The DSN tracking stations are located at
Canberra, Australia; Goldstone, Calif.; and Madrid, Spain.
The GN stations are equipped with a wide variety of tracking and
data-gathering antennas, ranging in size from 14 to 85 feet in
diameter. Each is designed to perform a specific task, normally in a
designated frequency band, gathering radiated electronic signals
(telemetry) transmitted from spacecraft.
The communications hub for the GN is the Goddard-operated NASA
Communications Center (NASCOM). It consists of more than 2 million
miles of electronic circuitry linking the tracking stations and the
MCC at the Johnson Space Center. NASCOM has six major switching
centers to insure the prompt flow of data. In addition to Goddard
and JSC, the other switching centers are located at JPL, KSC,
Canberra and Madrid.
The system includes telephone, microwave, radio, submarine cable and
geosynchronous communications satellites in ll countries. It
includes communications facilities operated by 15 different domestic
and foreign carriers. The system also has a wide-band and video
capability. In fact, Goddard's wide-band system is the largest in
the world.
A voice communications system called Station Conferencing and
Monitoring Arrangement (SCAMA) can conference link up hundreds of the
220 different voice channels throughout the United States and abroad
with instant talk/listen capability. With its built-in redundancy,
SCAMA has realized a mission support reliability record of 99.6
percent. The majority of Space Shuttle voice traffic is routed
through Goddard to the MCC.
As would be expected, computers play an important role in GN
operations. They are used to program tracking antenna pointing
angles, send commands to orbiting spacecraft and process data which
is sent to the JSC and Goddard control centers.
Shuttle data is sent from the tracking network to the main switching
computers at GSFC. These are UNISYS 1160 computers which reformat
and transmit the information to JSC almost instantaneously at a rate
of l.5 million bits per second, via domestic communications
satellites.
Space Network. Augmenting the GN and eventually replacing it, is a
unique tracking network called the Space Network (SN). The
uniqueness of this network is that instead of tracking the Shuttle
and other Earth-orbiting spacecraft from a world-wide network of
ground stations, its main element is an in-orbit series of satellites
called the Tracking and Data Relay Satellite System (TDRSS), designed
to gather tracking and data information from geosynchronous orbit and
relay it to a single ground terminal located at White Sands, N.M.
The first spacecraft in the TDRS system, TDRS-1, was deployed from
the Space Shuttle Challenger on April 4, 1983. Although problems
were encountered in establishing its geosynchronous orbit at 41
degrees west longitude (over the northeast corner of Brazil), TDRS-l
proved the feasibility of the tracking station-in-space concept when
it became operational later in the year.
Ultimately, the SN will consist of three TDRS spacecraft in orbit,
one of which will be a backup or spare to be available for use if one
of the operational spacecraft fails. Each satellite in the TDRS
system is designed to operate for 10-years.
Following its planned deployment from the Space Shuttle Discovery
scheduled for the STS-26 mission, TDRS-2 will be tested and then
positioned in a geosynchronous orbit southwest of Hawaii at 171
degrees west longitude, about 130 degrees from TDRS-1. With these
two spacecraft and the White Sands Ground Terminal (and eventually a
backup terminal) operational, the SN will be able to provide almost
full-time communications and tracking of the Space Shuttle, as well
as for up to 24 other Earth-orbiting spacecraft simultaneously. The
global network of ground stations can provide only about 20 percent
of that coverage. Eventually some of the current ground stations
will be closed when the SN becomes fully operational.
After data acquired by the TDRS spacecraft are relayed to the White
Sands Ground Terminal, they are sent directly by domestic
communications satellite to NASA control centers at JSC for Space
Shuttle operations, and to Goddard which schedules TDRSS operations
including those of many unmanned satellites.
The TDRS are among the largest and most advanced communications
satellites ever developed. They weigh almost 5,000 lb. and measure
57 ft. across at their solar panels. They operate in the S-band and
Ku-band frequencies and their complex electronics systems can handle
up to 300 million bits of information each second from a single user
spacecraft. Among the distinguishing features of the spacecraft are
their two huge, wing-like solar panels which provide l,850 watts of
electric power and their two 16-ft. diameter high-gain parabolic
antennas which resemble large umbrellas. These antennas weigh about
50 lb. each.
The communications capability of the TDRSS covers a wide spectrum
that includes voice, television, analog and digital signals. No
signal processing is done in orbit. Instead, the raw data flows
directly to the ground terminal. During Space Shuttle missions,
mission data and commands pass almost continuously back and forth
between the orbiter and the MCC at JSC.
Like the TDRS, the White Sands ground terminal is one of the most
advanced in existence. Its most prominent features include three
60-ft.-diameter Ku-band antennas which receive and transmit data. A
number of smaller antennas are used for S-band and other Ku-band
communications.
Ground was broken in September 1987, for a second back-up ground
terminal at White Sands to accommodate increased future mission
support required from the TDRSS.
The TDRSS segment of the Space Network, including the ground
terminal, is owned and operated for NASA by CONTEL Federal Systems
Sector, Atlanta, Ga. The spacecraft are built the TRW Federal
Systems Division, Space and Technology Group, Redondo Beach, Calif.
TRW also provides software support for the White Sands facility.
The TDRS parabolic antennas are built by the Harris Corp's Government
Communications Systems Division, Melbourne, Fla. Harris also
provides ground antennas, radio frequency equipment and other ground
terminal equipment.
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FLIGHT OPERATIONS
The Space Shuttle, as it thunders away from the launch pad with its
main engines and solid rocket boosters (SRB) at full power, is an
unforgettable sight. It reaches the point of maximum dynamic
pressure (max Q) -- when dynamic pressures on the Shuttle are
greatest -- about 1 minute after liftoff, at an altitude of 33,600
ft. At this point the main engines are "throttled down," to about 75
percent, thus keeping the dynamic pressures on the vehicle's surface
to about 580 lb. per square foot. After passing through the max Q
region, the main engines are throttled up to full power. This early
ascent phase is often referred to as "first stage" flight.
Little more than 2 minutes into the flight, the SRBs, their fuel
expended, are jettisoned from the orbiter. The Shuttle is at an
altitude of about 30 miles and traveling at a speed of 2,890 miles an
hour. The spent SRB casings continue to gain altitude briefly before
they begin falling back to Earth. When the spent casings have
descended to an altitude of about 17,000 ft., the parachute
deployment sequence starts, slowing them for a safe splashdown in the
ocean. This occurs about 5 minutes after launch. The boosters are
retrieved, returned to a processing facility for refurbishment and
eventual reused.
Meanwhile, the "second stage" phase of the flight is underway with
the main engines propelling the vehicle ever higher on its ascent
trajectory. At about 8 minutes into the flight, at an altitude of
about 60 miles, main engine cut-off (MECO) occurs. The Shuttle is
now traveling at a speed of 16,697 mph.
After MECO, the orbiter and the external tank are moving along a
trajectory that, if not corrected, would result in the vehicle
entering the atmosphere about halfway around the world from the
launch site. However, a brief firing of the orbiter's two Orbital
Maneuvering System (OMS) thrusters changes the trajectory and orbit
is achieved. This takes place just after the external tank has been
jettisoned and while the orbiter is flying "upside down" in relation
to Earth.
The separated external tank continues on a ballistic trajectory and
enters the Earth's atmosphere to break up over a remote area of the
Indian Ocean. Meanwhile, an additional firing of the OMS thrusters
places the orbiter into its planned orbit, which can range from 115
to 600 miles above the Earth.
There are two ways in which orbit can be accomplished. These are
the conventional OMS insertion method called "standard" and the
direct insertion method.
The OMS insertion method involves a brief burn of the OMS engines
shortly after MECO, placing the orbiter into an elliptical orbit. A
second OMS burn is initiated when the orbiter reaches apogee in its
elliptical orbit. This brings the orbiter into a near circular
orbit. If required during a mission, the orbit can be raised or
lowered by additional firings of the OMS thrusters.
The direct insertion technique uses the main engines to achieve the
desired orbital apogee, or high point, thus saving OMS propellant.
Only one OMS burn is required to circularize the orbit, and the
remaining OMS fuel can then be used for frequent changes in the
operational orbit, as called for in the flight plan.
The first direct insertion orbit was accomplished during the STS
41-C mission in April 1984, when the Challenger was placed in a
288-mile-high circular orbit where its flight crew was able to
successfully capture, repair and redeploy a free-flying spacecraft,
the Solar Maximum satellite (Solar Max) -- an important "first" for
the Space Shuttle program.
Launch Abort Modes. During the ascent phase of a Space Shuttle
flight, if a situation occurs that puts the mission in jeopardy --
the loss, for example, of one or more of the main engines or the OMS
thrusters -- the mission may have to be aborted. During the ascent
phase, there are two basic Shuttle abort modes: intact aborts and
contingency aborts. NASA has attempted to anticipate all possible
emergency situations that could occur, and mission plans are prepared
accordingly.
Intact aborts -- there are four different types -- permit the safe
return of the orbiter and its crew to a pre-planned landing site.
When an intact abort is not possible, the contingency abort option
becomes necessary. This crucial abort mode is designed to permit
crew survival following a severe systems failure in which the vehicle
is lost. Generally, if a contingency abort becomes necessary, the
damaged vehicle would fall toward the ocean and the crew would
exercise escape options that were developed in the aftermath of the
Challenger accident. The four intact abort modes are:
Return to Launch Site (RTLS)
Trans-Atlantic Abort Landing (TAL)
Abort Once Around (AOA)
Abort to Orbit (ATO)
Since an intact abort could result in an emergency landing, before
each flight, potential contingency landing sites are designated and
weather conditions at these locations are monitored closely before a
launch. Space Shuttle flight rules include provisions for minimum
acceptable weather conditions at these potential landing sites in the
event of intact abort is necessary.
In an abort situation, the type and time of the failure determines
which abort mode is possible. There is a definite order of
preference for an abort. In cases where performance loss is the only
factor, the preferred modes would be ATO, AOA, TAL or RTLS, in that
order. The mode selected normally would be the highest preferred one
that can be completed with the remaining vehicle performance.
In the case of an extreme system failure -- the loss of cabin
pressure or orbiter cooling systems -- the preferred mode would be
the one that would terminate the mission as quickly as possible.
This means that the TAL or RTLS modes would be more preferable than
other modes.
An ascent abort during powered flight can be initiated by turning a
rotary switch on a panel in the orbiter cockpit. The switch is
accessible to both the commander and the pilot. Normally, flight
rules call for the abort mode selection to be made by the commander
upon instructions from the Mission Control Center. Once the abort
mode is selected, the on board computers automatically initiate abort
action for that particular abort.
A description of the intact abort modes follows.
RETURN TO LAUNCH SITE (RTLS). The RTLS abort is a critical and
complex one that becomes necessary if a main engine failure occurs
after liftoff and before the point where a TAL or AOA is possible.
RTLS cannot be initiated until the SRBs have completed their normal
burn and have been jettisoned. Meanwhile, the orbiter with the
external tank still attached continues on its downrange trajectory
with the remaining operational main engines, the two OMS and four aft
RCS thrusters firing until the remaining main engine propellent
equals the amount needed to reverse the direction of flight and
return for a landing. A "pitch-around" maneuver of about 5 degrees
per second is then performed to place the orbiter and the external
tank in an attitude pointing back toward the launch site. OMS fuel
is dumped to adjust the orbiter's center of gravity.
When altitude, attitude, flight path angle, heading, weight, and
velocity/range conditions combine for external tank jettisoning, MECO
is commanded, and the external tank separates and falls into the
ocean. After this, the orbiter should glide to a landing at the
launch site landing facility. From the foregoing, it can be
appreciated why RTLS is the least preferred intact abort mode.
TRANS-ATLANTIC ABORT LANDING (TAL). The TAL abort mode is designed
to permit an intact landing after the Shuttle has flown a ballistic
trajectory across the Atlantic Ocean and lands at a designated
landing site in Africa or Spain. This abort mode was developed for
the first Shuttle launch in April 1981, and has since evolved from a
crew-initiated manual procedure to an automatic abort mode. The TAL
capability provides an abort option between the last RTLS opportunity
up to the point in ascent known as the "single-engine press to MECO"
capability --meaning that the orbiter has sufficient velocity to
achieve main engine cutoff and abort to orbit, even if two main
engines are shut down. TAL also can be selected if other system
failures occur after the last RTLS opportunity. The TAL abort mode
does not require any OMS maneuvers.
Landing sites for a TAL vary from flight to flight, depending on
the launch azimuth. For the first three Space Shuttle missions, the
trajectory inclination was about 28 degrees which made the U.S. Air
Force bases at Zaragoza and Moron in Spain, the most ideal landing
sites for TAL. Later Shuttle missions called for air fields at
Dakar, Senegal, and Casablanca, Morocco, as TAL-option landing sites.
In March 1988, NASA announced that in addition to the TAL sites in
Spain, that two new African contingency landing sites had been
selected for future Shuttle missions: a site near Ben Guerir,
Morocco, about 40 miles north of Marrakesh with a 14,000-foot runway;
and at Banjul, the capital of the west African nation of The Gambia,
which has an international airfield with an ll,800-foot runway.
ABORT ONCE AROUND (AOA). This abort mode becomes available about 2
minutes after SRB separation, up to the point just before an abort to
orbit is possible. AOA normally would be called for because of a
main engine failure. This abort mode allows the Shuttle to fly once
around the Earth and make a normal entry and landing at Edwards AFB,
Calif., or White Sands Space Harbor, near Las Cruces, N.M. An AOA
abort usually would require two OMS burns, the second burn being a
deorbit maneuver.
There are two different AOA entry trajectories. These are the
so-called normal AOA and the shallow. The entry trajectory for the
normal AOA, is similar to a normal end-of-mission landing. The
shallow AOA, on the other hand, results in a flatter entry
trajectory, which is less desirable but uses less propellant for the
OMS burn. The shallow trajectory also is less desirable because it
exposes the orbiter to a longer period of atmospheric entry heating
and to less predictable aerodynamic drag forces.
ABORT TO ORBIT (ATO). The ATO mode is the most benign of the
various abort modes. ATO allows the orbiter to achieve a temporary
orbit that is lower than the planned. ATO is usually necessary
because of a main engine failure. It places fewer performance
demands on the orbiter. It also gives ground controllers and the
flight crew time to evaluate the problem. Depending on the
seriousness of the situation, one ATO option is to make an early
deorbit and landing. If there are no major problems, other than the
main engine one, an OMS maneuver is made to raise the orbit and the
mission is continued as planned.
The first Space Shuttle program ATO occurred on July 29, 1985,
following the STS 51-F Challenger launch, when one of the main
engines was shut down early by computer command because of a failed
temperature sensor. Within 10 seconds of the shutdown, Mission
Control declared an ATO situation, and although a lower than planned
orbit was attained, the 7-day mission carrying Spacelab-2 was
successfully completed.
On-Orbit Operations. Space Shuttle flights are controlled by
Mission Control Center (MCC) -- usually referred to as "Houston" in
air to ground conversations.
During a flight, Shuttle crews and ground controllers work from a
common set of guidelines and planned events called the Flight Data
File. The Flight Data File includes the crew activity plan, payload
handbooks and other documents which are put together during the
elaborate flight planning process.
Each mission includes the provision for at least two crew members to
be trained for extravehicular activity (EVA). EVA is an operational
requirement when satellite repair or equipment testing is called for
on a mission. However, during any mission, the two crew members must
be ready to perform a contingency EVA if, for example, the payload
bay doors fail to close properly and must be closed manually, or
equipment must be jettisoned from the payload bay.
The first Space Shuttle program contingency EVA occurred in April
1985, during STS 51-D, a Discovery mission, following deployment of
the SYNCOM IV-3 (Leasat 3) communications satellite Leasats'
sequencer lever failed and initiation of the antenna deployment and
spin-up and perigee kick motor start sequences did not take place.
The flight was extended 2 days to give mission specialists Jeffrey
Hoffman and David Griggs an opportunity to try to activate the lever
during EVA operations which involved using the RMS. The effort was
not successful, but was accomplished on a later mission.
Each Shuttle mission carries two complete pressurized spacesuits
called Extra Vehicular Mobility Units (EMU) and backpacks called
Primary Life Support Systems (PLSS). These units, along with
necessary tools and equipment, are stored in the airlock off the
middeck area of the orbiter, ready for use if needed.
As already mentioned, for each mission, two crew members are trained
and certified to perform EVAs, if necessary. For those missions in
which planned EVAs are called for, the two astronauts receive
realistic training for their specific tasks in the Weightless
Environment Training Facility at Johnson, with its full-scale model
of the orbiter payload bay.
Maneuvering in Orbit. Once the Shuttle orbiter goes into orbit, it
is operating in the element for which it was designed: the near
gravity-free vacuum of space. However, to maintain proper orbital
attitude and to perform a variety of maneuvers, an extensive array of
large and small rocket thrusters are used -- 46 in all. Each of
these thrusters, despite their varying sizes, burn a mixture of
nitrogen tetroxide and monoethylhydrazine, an efficient but toxic
combination of fuels which ignite on contact with each other.
The largest of the 46 control rockets are the two Orbital
Maneuvering System (OMS) thrusters which are located in twin pods at
the aft end of the orbiter, between the vertical stabilizer and just
above the three main engines. Each of the two OMS engines can
generate 6,000 lb. of thrust. They can cause a more than l,000
foot-per-second change in velocity of a fully loaded orbiter. This
velocity change is called Delta V.
A second and smaller group of thrusters make up the Reaction Control
System (RCS) of which there are two types: the primaries and the
verniers. Each orbiter has 38 primary trusters, 14 in the forward
nose area and 12 on each OMS pod. Each primary thruster can generate
870 lb. of thrust. The smallest of the RCS thrusters, the verniers,
are designed to provide what is called "fine tuning" of the orbiter's
attitude. There are two vernier thrusters on the forward end of the
orbiter and four aft, each generates 24 pounds of thrust.